SAS 6G vs. 3G for controllers/disks

[Cory_K]

Hello Disk Experts!

A few questions for you regarding SAS 6G vs. 3G. I'm using a RAID 5 configuration, so please keep this in mind for the below.


From what I understand, a spinning hard drive (vs. SSD) will never hit anywhere near the limit for 3G speed for sustained read/writes.

1) Is there any reason to get SAS 6G spinning hard drives, and if so, in what situations and why?


From what I can gather, and these are rough numbers from Seagate's Cheetah drives, a SAS 10K drive can push out a sustained 88MB/sec/drive, and a SAS 15K drive can do about 120MB/sec/drive.

If a 3G controller can maintain it's theoretical 3G throughput (doubtful), then I would need at least four SAS 10K drives or three 15K SAS drives before I could possibly see any real benefit to 6G.

2) Is this logic flawed, and if so, how?

3) What is the maximum sustained throughput on a 6G controller, e.g., the HP P410 (462862-B21)?

4) In what situations does it make sense to use a 6G controller with 3G drives?


Many thanks!

[Cass Witkowski]

When looking at how long a disk I/O takes you have the time to seek but also the time to transfer the data from the disk to the controller. 3 Gb/s gives you a maximum theoretical transfer rate of ~300 MB/s. a 6Gb/s link should double that to ~600 MB/s

If you are doing I/Os with large transfer size say 1 MB (MegaByte) then the time it takes for one transfer is 3.3 ms at 3 Gb/s and 1.7 ms at 6 Gb/s. Assuming an average seek time of 3.5 ms then an I/O would take about 6.8 for 3 Gb and 5.2 ms for 6 Gb drives and controllers. This timing is greatly simplified. This give you a I/O rate of 147 I/Os per second for the 3 Gb and 192 I/Os per second for the 6 Gb drive and controller.

Of course if your data size is much smaller then the difference would be smaller. Looking at an 8K transfer size we get an I/O rate of 283 and 284 I/O per second for 3 Gb and 6 Gb respectively.

With RAID5 you also need to look at time to rebuild the raid set after a failure. While the raid set is rebuilding you can experience significant performance reductions. Again a faster transfer time will reduce this time.

Hope this helps